The description relates to a method and a device for managing priority during the transmission of a message in an interconnections network.
Interconnections networks ensure transmission of messages between various agents or entities of a communication network. A transmission may be made without performing message processing, or while performing message processing. In all cases one speaks of transmission of messages. Processing of a message is understood to mean for example an analysis of data contained in the message, or an appending of data to the message.
A message is, for example, a string of computer data, that is to say a string of bits or bytes. Each message includes a message header which chiefly includes the destination address of the message and the size of the message.
These interconnections networks are generally composed of two distinct parts which interact, namely a hardware part including physical links and a network controller, hard-wired or microprogrammed, which executes a low-level protocol, and a software part for effecting the interface with software functions of higher level.
An ideal interconnections network would certainly be a fully interconnected network, that is to say a network in which each pair of agents is coupled by a point-to-point link. However, this is unrealistic onwards of a few tens of agents. It is therefore necessary for the interconnections network to be able to cater for all the communications between agents with a limited number of links per agent.
Specialized agents exist for performing routing or transmission of messages flowing over the interconnections network.
These interconnections networks include transmission devices or routers (also known as “switches”), an organization of the network ensuring the link between the routers and the other agents, and a routing assembly, which caters for the flow of messages within the organization of the network.
A router is an active agent of the interconnections network which receives as input messages coming from one or more agents and which forwards or routes each of these messages respectively to their destination agent or to another router. This routing is performed by means of the address of the message destination agent, which is present in the header of the message to be routed.
The organization of a network constitutes the physical structure linking the various nodes or points of connections of an interconnections network.
The routing assembly manages the way in which a message is routed, or forwarded, from a source agent dispatching the message to a message destination agent through routers, along a routing path.
Any agent present in an interconnections network can dispatch and/or receive messages. The routing technique determines the way in which the messages are transmitted from the dispatching agent, which creates the message, to the destination agent. For a given interconnections network, there are various routing techniques, for which the main objectives are to reduce the end of message latency, or end of message forwarding time, for a message, from the dispatching agent to the destination agent, to increase the overall throughput and to improve the overall reliability of the network. The latency includes all the waiting times due to the way in which the messages propagate in the network and more particularly through the devices for transmitting messages in charge of routing the messages. The throughput is the quantity of computer data that a link of the network can transport per unit time, and can be measured locally over a link of the interconnections network or globally over the whole of the interconnections network. The reliability of a network is important, since the probability of errors grows rapidly with the number of nodes in an interconnections network.
A transmission agent includes at least one input including a means of storage organized as a message queue of the first in first out or FIFO type and at least one output including a message transmission means able to decide on the message to be transmitted. Only the first messages of the queues of the inputs are, of course, visible to the transmission agent.
Routing methods taking account of priorities between messages exist. The document present on the site brocade.com from the company Brocade, entitled “Providing reliable, high speed operations in large SANs” and the document present on the site cva.stanford.edu entitled “Virtual Channel, Flit Reservation Flow Control and Deadlock” talk of a procedure consisting in assigning priorities to messages during their creation, and in ordering the messages of a message queue of an input of a transmission agent according to their priority.
This modification of the order of the messages has a high hardware and software cost. Moreover, certain applications do not support the feature of messages not arriving in the order in which they were created, like certain real-time applications, for example display or audio or video retransmission applications.